Reprocessing method by fluoride volatility process using solid-gas separation

ABSTRACT

Fluorine or a fluorine compound is subjected to a reaction with a spent oxide fuel to produce fluorides of uranium and plutonium, and the fluorides are recovered using a difference in volatility behavior. The spent oxide fuel is subjected to a reaction with an HF gas, whereby uranium, plutonium and most impurities are converted into solid fluorides having low valences or remained as oxides to inhibit volatilization thereof, and then in an F 2  fluorination step, the HF fluorination product is subjected to a reaction with a fluorine gas in two stages: one at a low temperature and the other at a high temperature, whereby a certain amount of gaseous uranium and volatile impurities are separated with plutonium kept in a solid form in the first stage, and mixed fluorides of remaining uranium and plutonium are fluorinated into hexafluorides at the same time in the second stage. By such a reprocessing method, plutonium enrichment can be adjusted, uranium and plutonium can be purified, and steps are simplified as well. In addition, reactors are hard to be corroded or deteriorated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of separating and purifying nuclear fuel substances using a difference in fluorination volatility behavior of uranium, plutonium and other elements in a spent oxide fuel to reprocess the spent oxide fuel.

2. Description of the Related Art

A fluoride volatility process is one of methods for dry reprocessing of a spent fuel, in which nuclear fuel substances such as uranium and plutonium and various kinds of nuclear fission products are separated and recovered using a difference in volatility behavior when they are fluorinated. Techniques for applying the fluoride volatility process to a reprocessing process have been developed in the U.S. and other various countries since 1950s. However, each of those techniques has problem in higher fluorination and purification of plutonium. None of these techniques has reached a practical phase, and there has been no progress since 1970s.

In Japan, Japan Atomic Energy Research Institute has carried out the fluoride volatility process, and many advantages have been found, but development of the technique has been ended without establishing a plutonium purification step. In the fluoride volatility process carried out in that institute, uranium and plutonium are separated by two-stage fluorination using a fluidized bed furnace as a reactor with the temperature and fluorine concentration being changed. For example, in the first stage, uranium is fluorinated with an F₂ concentration of 20% at the operating temperature of 330° C., and in the second stage, plutonium is fluorinated with an F₂ concentration of 100% at the operating temperature of 330 to 550° C.

However, there is a disadvantage that it takes much time to convert uranium into uranium hexafluoride (UF₆) because the reaction temperature in “fluorination of uranium” of the first stage is low. Further, in “fluorination of plutonium” of the second stage, there is a disadvantage that plutonium is hard to be converted into plutonium hexafluoride (PuF₆) (the conversion ratio or conversion rate decreases) from the point of view of thermodynamics and reaction temperature because plutonium forms into PuF₄ of an intermediate fluoride in the first stage, and the fluorine concentration is so high that incomplete fluidization easily occurs.

Thus, a reprocessing process according to a fluoride volatility process using a flame furnace as a reactor has been proposed (see, for example, Japanese Patent Laid-Open Specification No. 2001-153991). Unlike the fluidized bed furnace, the flame furnace is a reactor operating under conditions of high temperature and high fluorine gas atmosphere.

If plutonium is converted under the conditions, a direct fluorination reaction of PuO₂+3F₂ (or 6F)→PuF₆+O₂ occurs, and therefore PuF₄ is never produced as an intermediate fluoride. In addition, because the fluorination temperature and fluorine concentration are high, decomposition reaction of PuF₆ is hard to proceed. However, since a corrosive gas at high temperatures and in high concentrations is required for the reaction, and conversion conditions are severe, the reactor is easily corroded and deteriorated, thus causing a problem in terms of materials. In addition, there are disadvantages that temperature adjustment for conversion conditions of a target substance is impossible, a large amount of expensive fluorine gas is used, and so on.

SUMMARY OF THE INVENTION

Problems to be solved by the present invention are to establish a method making it possible to adjust the plutonium enrichment and purify uranium and plutonium by volatilizing and separating a predetermined amount of uranium, to simplify steps such as separation as much as possible, to establish a method for producing PuF₆ in which the material of a reactor is harder to be corroded, to reduce a consumption of expensive fluorine gas, and so on.

According to the present invention, there is provided a reprocessing method by a fluoride volatility process using solid-gas separation in which fluorine or a fluorine compound is subjected to a reaction with a spent oxide fuel to produce fluorides of uranium and plutonium, and recover uranium and plutonium as the fluorides using a difference in volatility behavior, the reprocessing method comprising the steps of:

HF fluorination of the spent oxide fuel in the reaction thereof with a hydrogen fluoride gas, thereby converting uranium and plutonium compounds and most impurities in the spent fuel into nonvolatile solid fluorides or oxides having low valences to inhibit volatilization thereof; and

F₂ fluorination of the HF fluorination product in the reaction thereof with a fluorine gas which reaction is controlled in two stages: one at a lower temperature and the other at a higher temperature, thereby separating an arbitrary amount of gaseous uranium hexafluoride and volatile impurities with plutonium kept in a solid form in the first stage, and fluorinating the mixed fluorides of the remaining uranium and plutonium into hexafluorides at the same time in the second stage.

As described above, the present invention is characterized in that the spent oxide fuel is subjected to a reaction with a hydrogen fluoride gas having a weak oxidizing power, whereby only UF₄ (solid) is produced for uranium compounds, and only PuF₄ (solid) is produced for plutonium compounds to inhibit volatilization, and plutonium is separated from uranium and volatile impurities by solid-gas separation taking advantage of the characteristic of PuF₄ that it is hard to undergo a reaction into a fluoride having a high valence (higher fluoride).

In the reprocessing method, a raw material for uranium enrichment can be produced by trapping purified UF₆ alone with at least one cold trap, and a raw material for MOX (mixed oxide) fuel can be produced by trapping an enrichment adjusted-mixture of UF₆ and PuF₆ with at least one cold trap.

In the reprocessing method by the fluoride volatility process using the solid-gas separation according to the present invention, the spent oxide fuel is subjected to a reaction with hydrogen fluoride to consciously produce PuF₄ hard to undergo a reaction into a higher fluoride as an intermediate fluoride in the HF fluorination step so that the plutonium fluoride remains as a solid in a furnace, and then UF₄ is converted into UF₆ (gas) with PuF₄ unchanged in the first stage of F₂ fluorination, and therefore plutonium can be separated from uranium and the like in the F₂ fluorination step, the step is simplified, and the plutonium enrichment can be adjusted. In addition, since fluidized bed furnaces are used to carry out reactions under lenient conditions in both the HF fluorination step and F₂ fluorination step, the bed furnaces are hard to be corroded or deteriorated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing a basic process of a reprocessing method by a fluoride volatility process using solid-gas separation according to the present invention;

FIG. 2 is a process flow showing one example of the reprocessing method by the fluoride volatility process using solid-gas separation according to the present invention; and

FIG. 3 is a block diagram of an apparatus for carrying out the reprocessing method by the fluoride volatility process using solid-gas separation according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is an explanatory view showing a basic process of a reprocessing method by a fluoride volatility process using solid-gas separation according to the present invention. This is a reprocessing method in which fluorine or a fluorine compound is subjected to a reaction with a spent oxide fuel to produce fluorides of uranium and plutonium, and recover uranium and plutonium as UF₆ and UF₆+PuF₆ using a difference in volatility behavior. The method of the present invention comprises an HF fluorination step and an F₂ fluorination step in two stages of different temperatures.

In the HF fluorination step, a spent oxide fuel containing UO₂ and PuO₂ is subjected to a reaction with a hydrogen fluoride gas, whereby uranium, plutonium and most impurities in the spent fuel are converted into solid fluorides or oxides to inhibit volatilization. The HF fluorination step is carried out by supplying a hydrogen fluoride gas (supplied amount: 1.1 to 1.3 times the stoichiometric ratio, concentration: 60 to 90 vol %) using a fluidized bed furnace operated in the temperature range of 350 to 430° C. Since the hydrogen fluoride gas has a weak oxidizing power, a nonvolatile fluoride having a low valence (lower fluoride) is produced by the HF fluorination reaction. Only UF₄ (solid) is produced for uranium compounds, and only PuF₄ (solid) is produced for plutonium compounds. Impurities can be made to remain in the furnace as solid fluorides or oxides except for tellurium difluoride (TeF₂). For the hydrogen fluoride gas supply, an optimum amount was found to be 1.15 times the stoichiometric ratio from thermodynamic calculation and empirically, and a hydrogen fluoride gas supply which is 1.1 to 1.3 times the stoichiometric ratio brought about a preferred result. The used amount of expensive fluorine gas can be reduced by about 67% compared to conventional methods because an HF gas is used in the HF fluorination.

In the first stage of the F₂ fluorination step, the HF fluorination product is subjected to a reaction with a fluorine gas at a relatively low temperature, whereby uranium is converted into UF₆ (gas) with plutonium kept in the form of PuF₄ (solid), and plutonium is separated from a part of uranium using solid-gas separation. The first stage F₂ fluorination step is carried out by supplying a fluorine gas (supplied amount: 1.1 to 1.3 times the stoichiometric ratio of UF₄) diluted to 20 to 40 vol % using a fluidized bed furnace operated in the temperature range of 350 to 450° C. At this time, the plutonium enrichment in the furnace can be adjusted by volatilizing a predetermined amount of uranium. Moreover, a large amount of impurities is volatilized, and therefore a mixture of uranium and plutonium remaining in the furnace can be purified.

In the second stage of the F₂ fluorination step, the remaining product of the first stage of the F₂ fluorination is subjected to a reaction with the fluorine gas at a relatively high temperature, whereby mixed fluorides of remaining uranium and plutonium are fluorinated into hexafluorides at the same time. The second stage F₂ fluorination step is carried out by supplying a fluorine gas (supplied amount: 1.1 to 1.3 times the stoichiometric ratio of all fluorides remaining in the furnace) diluted to 20 to 40 vol % using the fluidized bed furnace operated in the temperature range of 730 to 800° C. At this time, by increasing the fluorine gas supply, the stability and production rate of PuF₆ can be improved.

In this way, the fluoride volatility process can be applied to recover uranium and plutonium as UF₆ and UF₆+PuF₆. By selectively condensing purified UF₆ alone, or the UF₆.PuF₆ mixture after adjustment of the enrichment, UF₆ and UF₆+PuF₆ can be used for producing a raw material for uranium enrichment (light water reactor nuclear fuel cycle) and a raw material for MOX (mixed oxide) (FBR nuclear fuel cycle). If the processing object is a spent metal fuel, the method of the present invention can be applied by oxidizing the metal fuel as preprocessing.

EXAMPLES

FIG. 2 is a process flow showing one example of the reprocessing method by the fluoride volatility process using solid-gas separation according to the present invention. This is an example of a process of reprocessing a spent oxide fuel. The spent oxide fuel as a raw material has been subjected to decladding processing, and its main constituent elements include U, Pu, O, Zr, Nb, Mo, Tc, Ru, Sb, Te, Cs, Np, Am and Cm, and uranium exists in a form of UO₂ and plutonium exits in a form of PuO₂. These raw materials are fluorinated in two stages.

<HF Fluorination>

In HF fluorination, a raw material (spent oxide fuel) is subjected to a reaction with an HF gas (supply: 1.15 times the stoichiometric ratio, concentration: 70 vol %) using a fluidized bed furnace (operating temperature: 400° C.). Consequently, UF₄ and PuF₄ are produced. Most impurities in the raw material form into fluorides or oxides. Resulting reactions are as shown below. PuO₂ (solid)+4HF═PuF₄ (solid)+2H₂O UO₂ (solid)+4HF═UF₄ (solid)+2H₂O <First Stage F₂ Fluorination>

In the first stage F₂ fluorination, a partial intermediate fluoride produced by HF fluorination is converted into a hexafluoride. The operating temperature of the fluidized bed furnace is set to 420° C., and the HF fluorination product is subjected to a reaction with a fluorine gas to produce a hexafluoride of uranium. The fluorine gas to be supplied is diluted with an N₂ gas to set the concentration to 20 to 40 vol %, and the supplied amount is set to 1.15 times the stoichiometric ratio required for conversion of UF₄ into UF₆. By this reaction, uranium is converted into UF₆ (gas). Many impurities form into volatile fluorides along with UF₆. However, most plutonium does not change from the form of PuF₄, and therefore by appropriately controlling the reaction for producing UF₆, the plutonium enrichment can be adjusted in the fluidized bed furnace. UF₄+F₂═UF₆ (gas, at 420° C.) <Second Stage F₂ Fluorination>

In the second stage F₂ fluorination, the operating temperature of the fluidized bed furnace is increased to 750° C., and the remaining product of the first stage of the F₂ fluorination is subjected to a reaction with a fluorine gas to produce hexafluorides of uranium and plutonium. The fluorine gas to be supplied is diluted with an N₂ gas to set the concentration to 20 to 40 vol %, and the supplied amount is set to 1.15 times the stoichiometric ratio required for highly fluorinating all fluorides and oxyfluorides (except for a bed material) remaining in the furnace. By this reaction, uranium is converted into UF₆ (gas) and plutonium is converted into PuF₆ (gas). By this operation, nonvolatile substances such as ZrF₄, CsF, AmF₃ and CmF₃ remain along with the bed material. By increasing the fluorine gas supply, the stability of PuF₆ can be improved and the conversion ratio can be increased. For example, if fluorine is supplied in an amount 6.8 times the stoichiometric ratio, the conversion ratio is about 40%.

<Refinement of UF₆>

UF₆ produced by the first stage F₂ fluorination contains a very small amount of PuF₆ and volatile impurities. These impurities are made to pass through chemical traps filled with a substance having an action of chemically adsorbing the impurities, whereby the impurities can be removed to purify UF₆. Chemical traps are installed in multiple stages as required.

(1) LiF—UO₂ F₂ Traps

In this process, chemical traps using lithium fluoride (LiF) and uranyl fluoride (UO₂F₂) either alone or in combination as a filler are used in the first stage for the purpose of removal of PuF₆. Instead thereof, any filler having an action of reducing PuF₆ may be used, and use of UF₄ and UF₅ can be considered as well. The reaction between LiF and PuF₆ is a reversible reaction, PuF₆ is adsorbed to LiF at the operating temperature of 300° C., and desorbed from LiF by heating it to 450° C. Thus, LiF is used at the operating temperature of 300° C. CsF is adsorbed at the temperature, too. PuF₆ adsorbed to UO₂F₂ can be used in an MOX (mixed oxide) reconverting process as a raw material without being desorbed.

(2) MgF₂ Traps

Chemical traps using MgF₂ as a filler are provided in the second stage. MgF₂ has an action of adsorbing NbF₆, MoF₆, TcF₆, RuF₅, SbF₅ and NpF₆, and is used at the operating temperature of 120° C. here.

(3) NaF Traps

Chemical traps using NaF as a filler are installed in the third stage. NaF is known to adsorb UF₆through the reaction of UF₆+2NaF→Na₂ UF₈ at 25 to 250° C. Na₂UF₈ decomposes into NaF and UF₆ again at 300 to 400° C., but NaF forms double salts with RuF₅ and NbF₆ at the temperature. It has an action of adsorbing ZrF₄, but most of ZrF₄ remains in the fluidized bed furnace as a nonvolatile substance along with the bed material, and only a very small amount thereof is removed by the NaF traps.

<Adjustment of Pu Enrichment>

A mixed gas of volatilized UF₆ and PuF₆ and UF₆ purified by the chemical traps are mixed together in a desired ratio using a gas mixer, and the plutonium enrichment is adjusted. For operating conditions of the gas mixer, a slightly negative pressure and a temperature of about 70 to 80° C. are adopted.

<Condensation of UF₆/UF₆.PuF₆>

UF₆ not used for adjustment of the plutonium enrichment, of UF₆ purified by the chemical traps, or a mixed gas of UF₆ after adjustment of the plutonium enrichment and PuF₆ is condensed. The operating temperature is −50 to −30° C., and the pressure is about 50 kPa.

<Volatilization of UF₆/UF₆.PuF₆>

The temperature is increased to 70 to 80° C. with the pressure (50 kPa) unchanged, whereby the condensed UF₆ or mixture of UF₆ and PuF₆ is vaporized and provided for a reconversion process.

If vaporized simple UF₆ is filled in a cylinder for UF₆, it can be used as a raw material for uranium enrichment, and this process can be used in a light water reactor fuel cycle. As an option, by setting conditions for liquefying UF₆, in which the temperature of the cold traps trapping UF₆ is 64° C. or higher and the pressure is 152 kPa or greater, UF₆ can be liquefied and filled in the cylinder.

FIG. 3 is a block diagram of an apparatus for carrying out the above reprocessing method by a fluoride volatility process using fractional distillation. A raw material (spent oxide fuel) in a raw material supply tank 10 is fed to an HF fluorination furnace (fluidized bed furnace) 12, reacts with an HF gas into an intermediate material, and is stored in an intermediate fluoride supply tank 14. The intermediate fluoride in the intermediate fluoride supply tank 14 is fed to an F₂ fluorination furnace (fluidized bed furnace) 16, and reacts with an F₂ gas into a hexafluoride.

UF₆ obtained by the first stage F₂ fluorination is purified through LiF/UO₂ F₂ traps 20, MgF₂ traps 22 and NaF traps 24, and introduced into cold traps 26. UF₆ and UF₆.PuF₆ obtained by the second stage F₂ fluorination are also introduced into the cold traps 26. In the cold traps 26, UF₆ is condensed and volatilized and UF₆.PuF₆ is condensed and volatilized, and they are provided for the reconversion process. 

1. A reprocessing method by a fluoride volatility process using solid-gas separation in which fluorine or a fluorine compound is subjected to a reaction with a spent oxide fuel to produce fluorides of uranium and plutonium, and recover uranium and plutonium as the fluorides using a difference in volatility behavior, the reprocessing method comprising the steps of: HF fluorination of the spent oxide fuel in the reaction thereof with a hydrogen fluoride gas, thereby converting uranium and plutonium compounds and most impurities in the spent fuel into nonvolatile solid fluorides or oxides having low valences to inhibit volatilization thereof; and F₂ fluorination of the HF fluorination product in the reaction thereof with a fluorine gas which reaction is controlled in two stages: one at a lower temperature and the other at a higher temperature, thereby separating an arbitrary amount of gaseous uranium hexafluoride and volatile impurities with plutonium kept in a solid form in the first stage, and fluorinating the mixed fluorides of the remaining uranium and plutonium into hexafluorides at the same time in the second stage.
 2. The reprocessing method by a fluoride volatility process using solid-gas separation according to claim 1, wherein a raw material for uranium enrichment is produced by trapping purified UF₆ alone with at least one cold trap, and a raw material for MOX (mixed oxide) fuel is produced by trapping an enrichment adjusted-mixture of UF₆ and PuF₆ with at least one cold trap. 